Masonry Wall Net Area vs. Equivalent Thickness
Masonry Wall Net Area vs. Equivalent Thickness
(OP)
It appears there is a discrepancy in calculating the net area of a masonry wall for axial stress (P/An) between the "Reinforced Masonry Engineering Handbook" (Amrhein, 1998) and the "Masonry Structures: Behavior and Design" text (Drysdale, 1999).
Looking at the RMEH on P.300, Table B-3a gives the "Equivalent Solid Thickness" for a 12" conc. wall grouted at 48"oc as being 6.5". In the footnotes, we see that fa = P/(EST)b, thus P/A. A in this case would be 6.5*12 = 78 in^2/ft.
Looking at the Drysdale text on P.873, Table B4, we see that An for Grouted Cores @ 48"oc gives An = 54.1 in^2/ft
Why do the two values differ so greatly? Which one should be used in the calculation of fa?
Looking at the RMEH on P.300, Table B-3a gives the "Equivalent Solid Thickness" for a 12" conc. wall grouted at 48"oc as being 6.5". In the footnotes, we see that fa = P/(EST)b, thus P/A. A in this case would be 6.5*12 = 78 in^2/ft.
Looking at the Drysdale text on P.873, Table B4, we see that An for Grouted Cores @ 48"oc gives An = 54.1 in^2/ft
Why do the two values differ so greatly? Which one should be used in the calculation of fa?






RE: Masonry Wall Net Area vs. Equivalent Thickness
I think if you look closely the 6.5 equivalent thickness is about the ungrouted or net area of the 12" masonry units.
In addition to ACI 530, The best place to get additional information is on the National concrete Masonry Association (ncma.org). On the upper right corner there is a place to click on.- Just click ANY state or supplier to get to the 110 TEK notes and go to the index and the structural section for the individual TEK notes you are interested in.
When the first ACI 530 was written, I remember Jim Amrhein and the NCMA staff engineers sorting things out. Drysdale was also involved in many meetings I attended.
Masonry is really designed by the wall panel and not the unit or materials (block, mortar, grout) since the individual materials are tested differently for different purposes.
RE: Masonry Wall Net Area vs. Equivalent Thickness
"I think if you look closely the 6.5 equivalent thickness is about the ungrouted or net area of the 12" masonry units."
In my 1978 edition if Amrhein, the term is "equivalent Solid Thickness" and is defined as ..."the calculated thickness of the wall if there were not hollow cores, and is obtained by dividing the volume of solid material by the face area if the wall." It was intended for the determination of area only for the structural design of the wall, as in specifically for shear and bearing.
My edition also shows the 6.5" figure for the same conditions in Table B-3a on page 278.
Mike McCann
MMC Engineering
RE: Masonry Wall Net Area vs. Equivalent Thickness
I am not aware of any codes that use the old methods and procedures, although the terms and methodology have been updated and adapted to the knowledge the old documents provides.
Just as reality check, the minimum ASTM 12x8x16 units give an approximate equivalent thickness for the unit itself of about 5.8 +or-. The determination of the volume of the units defy calculation (only accurately determined by water submersion) because of the radii, core tapers and various other items (some of which are structural attributes and others are functional/production based. Currently, the term "equivalent thickness" in testing (ASTM C140) is the volume of the unit divides by the actual face area (not including mortar). there may some othe standards that still use the older concepts and determination properties. NCMA TEKs 14-1B (2007 - Properties of Masonry Walls) and TEK 14.7B (2009 - Allowable Stress Design of Concrete Masonry) are several examples of information that is available in Chapter 14 of the TEK notes to use with the common standards.
In general, all design allowables are based on the prism strength (f'm) of a composite 2 high block/mortar hollow prism. Jim Amrheim was very sharp and understood the big picture and the difference is terms locally. In the 1980's, I showed him a series of test reports for 5000 psi 8" hollow prisms and the photographs of the testing and failed samples. It only took him a few seconds to spot the errors in the testing (which others noted later) that involved changes in the ASTM testing procedures that were ultimately corrected to eliminate the older testing and volume/linear dimensions.
How you look at the wall properties is determined by current standards and not older books and papers.
I am partial to ACI 530, since I was involved in the document and the fellow voting members. I have seem some international projects (10 -13 to20 story partially reinforced loadbearing buildings) that were completed with no job site prisms, a minimum of mortar tests and no clean-outs. the engineers said they used what they learned in Southern California in the later 1960's, early 1970's complimented by technology, good detailing, planning and technology. They said they used U.S. standards, but they used them better.
Dick
RE: Masonry Wall Net Area vs. Equivalent Thickness
RE: Masonry Wall Net Area vs. Equivalent Thickness
Mike McCann
MMC Engineering